The tenon is a square peg that protrudes from the end of a board and fits inside a corresponding mortise, a square hole, which is cut into another piece of wood. The mortise and tenon align workpieces as well as create a permanent joint between the two pieces of wood. Some production workshops use large shapers and table saws to cut mortise and tenons, but with a handheld router, you can accurately cut mortise and tenons in your own home workshop without using heavy equipment.
Place the router bit in the router and set the depth at 3/4-inch.
Place the guide on the router and set the guide at 1 inch to the centre of the router bit.
Clamp one of the pieces of poplar to a worktable. Make two marks on the poplar board 2 inches apart on the centre of the board. Set the router over the marks and push down, compressing the spring in the router and allowing the guide to flush with the side of the poplar. Check to see if the router bit is in the direct centre of the board. If it is not, adjust the guide on the router to centre the bit.
Turn on the router and slowly compress the spring in the router, letting the bit plunge into the wood. When the router base bottoms out on the poplar, slowly push the router between the lines that you have drawn on the poplar, creating a 2-inch mortise that is 3/4-inch deep.
Clamp the other piece of poplar to the table. Set the depth of the router bit to 1/8-inch. Set the router guide at 3/4-inch to the outside of the knife.
Set the router on the end of the piece of poplar to cut the tenon. Turn the router on, compress the spring in the router, bringing the bit down onto the poplar. Slide the router across the end of the poplar, cutting a 3/4-inch channel across the end of the poplar. It will leave a small ridge at the end of the board. Cut the ridge off with the chisel.
Release the clamp, turn the board over and put the clamp back on. Cut another channel across the end of the board opposite the first, and cut off the small remaining ridge with the chisel to create a 1/2-inch-thick tenon.
Test-fit the tenon into the mortise. Round the corners of the tenon with the chisel to fit snugly into the mortise.
Be careful when turning on a router. The torque can sometimes twist the router in your hands. Be careful when the bit engages the wood—the router can also kick if not held securely.